Compensating circuit



May 25,194s.^ D. D. GRIEG y 21,441,964

coMPENsA'rING CIRCUIT,

DONALD D. G/WE ffy/@f TTRNEY May 25, 1948. D. D. GRIEG 2,441,954,

COMPENSATING CIRCUIT Patented May 25, 1948 ooMrENsA'rING omoUrr Donald D. Grieg, Forest Hills, N. Y., assgnor to Federal Telephone and Radio Corporation, Ne York, N. Y., a corporation of Delaware Application May 19, 1944, Serial No. 536,302

2 Claims. 1

This invention relates to electrical circuits and more particularly to such circuits utilizing electron control tubes such as pulse modulator circuits. f

There are occasions when a relatively unstable source of voltage only is available for electrical apparatus such as a radio transmitter. This usually results from necessity rather than choice, as in the case off equipment for installation on airplanes or in :portable sets where the addition of voltage regulators would cause an undesirable or even prohibitive increase in weight. As a consequence of the random iiuctuations in the available voltage supply in such cases, the output of energy from the apparatus is likewise caused to vary, therebyv producing distortion in the speech or other intelligence being transmitted.

While a varying voltage supply of the above nature is undesirable in conventional amplitude modulation transmitters, such a condition becomes an even greater factor in pulse transmitters of the time modulation type. In the latter, any variation in the voltage supply results in an undesirable variation in pulse spacing or Iposition. Since the maximum time displacement of the Ipulses in time modulation systems is usually very small compared to the spacing between successive pulses, it will be clear that even a slight change in the amplitude of the voltage source will f cause a pulse time shift that is appreciable in comparison With the signal modulation and therefore undesirable.

lIt is accordingly anobject of the present Yinvention t0 provide means for-compensating for the effect of supply voltage fluctuations in circuits including electron discharge devices.

It is a further object of the invention to provide means for varying the modulator grid bias in an oscillator-modulator circuit of the pulse time modulator character in such a manner as t0 compensate'for variations in the common supply voltage of such circuits.

Other objects and advantages of the invention will be apparent from the following detailed description to be read in connection with the accompanying drawings, in which:

Fig. 1 is a circuit diagram of one form of time modulator incorporating the principles of the present invention;

Figs. 2A, 2B and 2C are curves usefulin explaining the operation of the circuit of Fig. l; and

Fig. 3 is a circuit diagram of a modification of the f form of the invention illustrated in Fig. l.

In Fig. 1 is shown a source of unregulated voltdivider 2 through a connnection 5. As a resulty of the oscillator 3 being supplied with plate voltage from source I. the output of the oscillator will vary in proportion to the value of the voltage supply, or, in other words, the voltage output of the oscillator 3 will vary linearly with respect to a variation in the voltage from source I.

A modulating circuit generally indicated at 6 is coupled to the oscillator 3 by means of trans- `former 'I having a primary coil 8 and two secondary coils 9 and I0. The primary coil 8 of the transformer 'l forms part of the plate tank circuit of the oscillator 3. The two secondary coils 9 and Il) have their outer terminals respectively connected to the grids II and I2 of two triodes I3 and I4, which as shown, may be enclosed in a common envelope. The plate voltage for the anodes of triodes I3 and I4 is supplied from an adjustable tap I5 on voltage divider 2 over a common connection I6. Y

The inner terminals of each of secondary windings 9 and I0 of transformer 'I are connected to the ends of the primary winding of an audio transformer Il. The midpoint of this primary Winding is grounded. The voltage for the speech circuit portion of the modulator 6, including microphone I8 and the secondary winding of transformer Il, is derived across voltage divider 2 by means of a third adjustable tap I9.

From the above description, it will be seen that a common voltage supply is provided for the oscillator 3, the plate circuit of modulator 6 and the audio circuit including microphone I8. Since a change in the potential of source I will result in a drop in the voltage across voltage divider 2, it can be seen that any' change in the supply voltage will produce proportional changes in the voltages taken off by adjustable taps 4, I5 and I9. Thus the voltages in the three above-mentioned circuits will at all times vary concurrently and in proportion to one another.

These corresponding variations compensate one for the other as regards the output of the circuit. A further compensating effect must be provided in order to prevent the variation of voltage source from aiecting` the circuit output. I accomplish this desired eiect by providing a biasing circuit which varies in response however do not to variation in the voltage source. The biasing circuit includes a cathode resistor connected between the cathode 2! of tube I4 and ground,V

a further resistor 22 connected between the upper terminal of resistor 2&1 and the cathode 23 of triode I3. A Kpair of conventional by-pass condensers 2li. and. zii respectively shunt, the resistor 2E! and 2,2- to ground.

The operation of the circuit disclosed in Fig. l

will be easily understood by considering the` curves of Figs. 2A, 2B and 2C. Each of the tri'- in windings 9 and El?, the shift in waves 23 and 29is simultaneously towardA or away from axis 34. This causes successive points 33, 3i, 32 and 33 to be displaced in opposite directions in pushmade upon the assumption that the voltage from odes I3, Ill is designed to have substantially ildentical operating characteristics, such as shown by the common characteristic ,operating curve-Biot Fig. 2A. It is preferable that the tubesv bedesigned to have a sharp cut-01T, such as the point 21.

The operating curve 26 VofV Fig. 2A repre-` sents the normal operating curve of tubes. I3 and.

I for a certain steady potential applied to the respective anodes thereof.V Resistors 2li and 22 are so chosen that the bias between grid l2 and cathodeZ'I oftriode Iii, on one hand, and thebia-s between Vgrid` Il and cathode 23 of triode I3, on the otherhand;willA be substantially equal on opposite sides of; the Vcut-ofpoint 2l' as indicated.v inrFig. 2A. Y

The wave produced by' oscillator 3' is applied through transformerI-to the twosecondary wind-V ings aand Hi.V This makes the outer-terminal of winding 9 180 out of phase with the outer Vterminal ofwindingI; Thus thewave-'of oscillatort, will be applied on grids; I-I; and I2 one inverted in phase with respect tothe other. 1

VDue to the difference in absolute bias between tubes I and I il, differentY portions' of the input waves to the grids IIV and i2 will ice-passed by the tubes. lf the wave input to tube'illis desig- V natedV as Z3 and the wave input to tube ri3V is,

designated asi-'29, thenY it will beseen that'waves 28; and 29- will intersect at unequallyispaced` points 3u, 3l",V 32 and 3,3; along a linear vertical aXisv 3ft which intersects the characteristiccurve,26; at crut-01TV point 2l. While; waves 2c and 29 areshown; for illustration purposesas triangular in shape, it; wi-,llbe understood, of course, that they will be sinusoidal from oscillator 3. Ii desired', the input waves may be made triangular by suitable-Shaper circuits.

Whenrthe two waves EliY and Euere projected` against the, characteristic curve the wave output-will be asrepresented in Fig; 2A. It will be noted that the spacing ofthe peaks'ilil, SI, 32 and source]V remains at a steady value. AssumeY now thatfor somereason this voltage undergoes a randomuctuation amounting to a drop of one-third in, absolute-valuathe bias on triodes I 3 andV It remaining Vas before.A The results of such a uctuatio'nV are? illustrated in Fig. 2B. As seen in this figure,-V the..characteristic curve of the triodesfIS and'Ili has dropped `from position 2t to a new position V(36. The amplitude ofV the, wave output from oscillator 3- has decreased proportionally, tha-tris,A to Vtwo-thirds of itswforinervalue. With the bia-ses'S'li` and 35 unchanged, the projection of waves 28a, 29a against the new operating curve is similar in the output curve of Fig. 2A as-was j the spacing of the corresponding points along the axis 3ft. If'desired, a suitable clipping and differentiating circuit can be provided in the output ofl modulator so that'the peaks at points Sii; 3l, 32 and 33 will be transformed into a train of pulses havingl the spacing representedby suchpoints. Where sinusoidal wavesare used at the input, the peaks are in the form of` cusps which when clipped provide sharp pulses;

36 will-'produce the output shown. Points 3&3, 3i, 3Z'Vand3g31wl1 assume new positions 36a, 3io, 32u and 33a; the amplitude ofV waves a'will be decreasedV considerably from its input value, and wave 2go will not be passed toany degree. As a result, the time displacement of points Sila, Ela, 32a and33a will varygconsiderably with respect to the-original spacingas shown in'Fig.' 2A. In other words, the outputofthe modulator 6 will be distorted' to a very considerable degree so as to reduce the delity of transmission.

Inorderto correct thedisrtortion resulting from the operation of' thecircuitof- Fig. 1 as shown in Fig; 2B, theresistors 2 0 and 22' employedV in connectiorr With the-triodes I3 and It are so'chosen that a ,ch-ange in plate current resulting fromY a changein voltage-fromvsource I will produce a dropacross resistor 2u thatis-proportional to this change-in currentf'or triode Hi and a proportional changein Voltage across; the resistor combination- 20, 22- fortriode I3.` Y

' The resultfof Yutilizing; such a combination oi resistors is illustrated-'in Fig. 2C. Fig. 2Cretains the newoperating curve 36 ci Fig. 2B, but the biases on triodes i3 andY I have each dropped one-thirdinvalue, 'and' have shifted to new position- 35u-and-v 31'611. It'should be emphasized that the change in the value of biasesu, 35a is proportional tothe'voltage fluctuation in source lila, and consequently tothe change in current in the plate circuit. Fig.r2C also illustrates theeffect of projecting-the"reduced amplitude waves 23a.` and 29d-Y symmetrically disposed on Ytheir new biases 34a and 35a. respectively, against the new operating curve 36. It will be seen that while the amplitude ci the resulting output sig- V nalis reduced the pointsof intersection of output Theaudio modulating 4circuit including microphone i8, land trai'isforrner i? is connected as abovedescribedto the inner Yterminals ofr each of secondary` coilsl Sand Iilof transformer I8-, By means oi" thisconnectiona variation'in the current through transformer H due `to the reception o'fjsoundl by microphone it will vary the current in the windings 9` and This` will cause theV wai/,e523 andil; o ijFig. to be, ined-ect, shifted relative to their respectivezero or bias 53d .and- 35, andthis in turn willi change the relative spacing betweenpulse positionoonts sa, 3i., 3.2; and, 3,3; As a result of the direction oil flow of" current waves 28d, 29a, are identical with those shown in Fig; 2A, that is pulse positionsy 3Q, 3l, 32 and 33 are time displaced Vin amount substantially equal vto that shofwnfinthe normal operation curve 2A. 'Y L A v Thus-it ls'seen that by the use of a compensating onszelf-regulatingarrangement including the Vresistors 2B and 22, it isrpossible toi maintain substantially constant the time displacement of Vthe pulse; positions 3Q; 3i, 3-2 andV 337 representing the points of intersection of theinput waves 28,Y andl 25S, thus avoidingV anyV distortion which might result in a shift in these points as shown incurve- 2B when compensating means is lacking.

Condensers 24 and 25 should be large enough to smooth pulsations in the cathode circuit across resistors 29 and 22.

Since the audio energy is supplied through adjustable tap I9, the audio modulation through transformer l1 will vary proportionally to the amplitude of the base wave from oscillator 3. Thus the relative modulation of the base wave by the audio wave will remain constant regardles-s of variation in voltage from source I.

While the variation in source potential has been shown in the curves of Figs. 2A, 2B and 2C as a voltage drop, it should be clearly understood that this has been selected merely as an example, and that the theory of operation is similar in all respects for an increase in voltage as for a decrease.

Fig. 3 illustrates a modification of Fig. l ein# ploying an additional resistor 37 connected between junction point 38 on lead I6 and the top terminal of resistor 22. This additional resistor 31 is of a high value, and acts to produce the desired bias voltage across resistors 2li and 22 independently of the operating conditions of tri` odes I3, I4., This modication of Fig. 3 maintains a constant D. C. potential across resistors 20, 22 regardless of the A. C. current through triodes I3 and I4. When the resistor 37 is utilized, a separate load resistor 39 is preferably employed between the output of the triodes and connection point 38.

While I have described above the principles of my invention in connection with specific apparatus, and particular modiiications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of my invention as set forth in the objects of my invention and the accompanying claims.

I claim:

l. In a time modulation system of the type including an oscillator and balanced modulator fed from a common source of unregulated voltage, in which the balanced modulator includes two electron discharge devices each having an anode, cathode and grid, a biasing arrangement for producing an initial differential bias on the two devices and While maintaining said `differential Varying the mean bias of said devices to compensate for variation in the Voltage of said source comprising a first biasing resistor connected in series between the cathode of one of said electron discharge devices and the negative -side of said source of potential, and a second biasing resistor connected at one end in series with said first resistor to the negative side of said source oi potential and at its other end to the cathode of the other of said electron discharge devices.

2. A time modulation system according to claim l, further including an arrangement for translating mechanical signals into electrical signals, means for coupling said arrangement in pushpull to the grids of said electron discharge devices, and a circuit for supplying said arrangement with potential from said source of potential.

DONALD D. GRIEG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

